FMR1 and fragile X syndrome: The condition has been estimated to affect ∼1:4000 males and 1:7000 females.3–5 Although both sexes are susceptible, males with FXS typically exhibit more severe symptoms compared with females because the single X chromosome in males is usually fully methylated and not producing FMRP6; females typically have some FMRP expression from the FMR1 gene on their second unaffected X chromosome.7 Males with FXS are far more likely than females to exhibit significant intellectual and developmental disabilities (85% vs. 25%).8